1. Field of the Invention
The present invention relates generally to an automatic transmission and more specifically to a hydraulic control circuit therefor which includes a shift shock attenuating arrangement.
2. Desription of the Prior Art
JP-A-61-165054 discloses a hydraulic transmission control system wherein a given shift valve is arranged to control the engagement of first and second friction elements. More specifically, this valve is arranged to upon switching to an upshift position, induce the engagement of one friction element and the release of the other. As the shift valve is arranged to supply hydraulic fluid to both of the friction elements in order to achieve the above engagement and release, an orifice and an accumulator are provided. The orifice is provided between the shift valve and the accumulator.
With this arrangement as the accumlator is used both during upshifting and downshifting an essentially constant pressure is maintained for the purposes of attenuating shift shock.
However, with this arrangement, the downshifting period tends to be prolonged and induces shift shock. That is to say, during downshifting the reduction in the pressure which is applied to the first friction element is delayed by the action of the accumulator and the time required for the completion of the shift exceeds the desired value. Further, at the point in time where the stroke of the accumulator terminates, a sudden reduction in hydraulic fluid pressure occurs and a momentary application of a large amount of torque induces a large shock.
One method of overcoming this problem is to reduce the fixed pressure level which is maintained by the accumulator. However, this measure induces the problem that the reduced level deteriorates the shift feeling during a 2-3 upshift.